1. Field of the Invention
This invention relates to a method for producing paper pulp containing fibers of annual plants or of secondary raw materials produced therefrom, the fibers being bleached, shortened and processed into a homogeneous paper pulp in further processing steps. The invention further relates to a method for producing shortened fibers of annual plants or of secondary raw materials produced from annual plants.
2. Description of the Related Art
For environmental reasons, there are ideas to grow and use annual plants, such as hemp, flax, etc., specifically for papermaking in order to reduce or even completely prevent deforestation at least for this branch of industry.
At present, papers from fibers of vegetable origin, such as cotton, linters, hemp, etc., are used mainly for high-quality papers, such as shares, securities, bank notes, etc., since the raw materials are expensive and the stock preparation of vegetable fibers is very elaborate. The relatively long fibers cannot be singled in water, i.e. they conglomerate and form large lumps. When rotational motions are added, as during stirring or pumping, these fiber conglomerations tend to and form meter-long braids. A special method therefore, had to be developed for the stock preparation of vegetable fibers.
In this known method, the fibers are pressed in a first step into so-called "cakes" and several of these cakes are suspended in a bleaching boiler. With the aid of bleaching chemicals, pressure and temperature, the fibrous material oxidizes to the desired whiteness. While the cakes are still in the bleaching boiler they are rinsed with water to remove the bleaching chemicals. The entire bleaching process takes several hours. Then the cakes are taken out, crushed and slowly supplied to a beater filled with water forming a fiber suspension. Here the fibers are shortened, likewise in a several-hour process. In a further method step, the fiber suspension now capable of being conveyed with conventional pumping systems is further shortened and fibrilled until, with the provision of additives necessary for papermaking, it reaches the paper machine.
Since the paper machine works continuously and new paper pulp must therefore be supplied constantly, while the stock preparation of the fibers only takes place discontinuously and in time-intensive single processes, it is necessary to use several bleaching boilers and beaters in parallel. This method is thus very elaborate and requires many individual separate method steps.